Network systems constituted by a master device and a slave device are used as network systems for industrial use, and techniques relating to such network systems have been disclosed.
For example, JP 2014-119907A discloses a communication unit communicably connected by a system bus to multiple slave devices that operate based on setting information. Upon determining that the system configuration is a configuration indicated by stored configuration information, the communication unit transmits setting information included in the configuration information to the slave devices. For this reason, JP 2014-119907A describes that the communication unit can prevent unexpected operations from occurring in the network system.
Also, JP H9-128019A discloses a remote I/O system including a remote master and multiple IN slaves and OUT slaves connected via a communication cable to the remote master. In the remote I/O system, the remote master collectively transmits OUT data to the slaves, and the OUT slaves receive the OUT data at respective node times. Meanwhile, based on the times at which the OUT data is received, the IN slaves sequentially transmit IN data at respective node times each time a basic idle period obtained with consideration given to frame length and physical delay elapses. The remote master receives the IN data from the IN slaves in time slots based on the basic idle period. For this reason, JP H9-128019A describes that overlapping of IN data due to physical delay in the communication cable or the like no longer occurs.
JP 2014-119907A and JP H9-128019A are examples of background art.
However, the above-described conventional techniques are problematic in that there are cases where establishing a link in the network system takes time when multiple slave devices are powered on at the same time, for example. This problem will be described with reference to FIGS. 8A and 8B. FIGS. 8A and 8B are diagrams showing communication processing in which the problem of two slave devices taking time to establish a link occurs in the conventional technology. FIG. 8A is a diagram showing a state in which link pulses collide, and FIG. 8B is a diagram showing times at which the two slave devices transmit link pulses.
In the network system shown in FIG. 8A, an OUT-side PHY (Physical Layer) unit of a slave device 101a and an IN-side PHY unit of a slave device 101b are connected. In this state, if the slave device 101a and the slave device 101b are powered on at the same time, the OUT-side PHY unit of the slave device 101a and the IN-side PHY unit of the slave device 101b both execute Auto MDI/MDI-X. In this case, as shown in FIG. 8B, if the OUT-side PHY unit of the slave device 101a and the IN-side PHY unit of the slave device 101b transmit link pulses at the same time, a link pulse collision occurs between the slave device 101a and the slave device 101b as shown in FIG. 8A. Note that Auto MDI/MDI-X is a known technique for network systems conforming to the Ethernet (registered trademark) standard, and thus description thereof is not included here.
If the slave device 101a and the slave device 101b cannot receive link pulses from each other for a predetermined amount of time, the link pulses are retransmitted. In this case as well, if the OUT-side PHY unit of the slave device 101a and the IN-side PHY unit of the slave device 101b retransmit the link pulses at the same time as shown in FIG. 8B, a link pulse collision occurs between the slave device 101a and the slave device 101b as shown in FIG. 8A.
Thus, if link pulses collide, the slave devices retransmit the link pulses, and this processing is repeated until a link is established. In particular, if the IN-side PHY units and OUT-side PHY units of the slave devices are provided with the same physical layer circuit, the times at which the link pulses are transmitted and retransmitted are likely to be the same, and link pulse collisions are more likely to occur. In this case, due to variation in the properties of oscillators and other components of the physical layer circuits and differences in temperature, the times at which the physical layer circuits retransmit the link pulses diverge from each other little by little and a link is ultimately established, but this takes time.